Summary

Significant differences in joint balance were observed between CPAK groups; A component alignment target based solely on pre-operative boney anatomy may not be sufficient to balance the joint.

Abstract

Introduction

The Coronal Plane Alignment of the Knee (CPAK) is a recent method for classifying knees using the hip-knee-ankle angle and joint line obliquity to assist surgeons in selection of an optimal alignment philosophy in total knee arthroplasty (TKA). This classification method however is based on bone geometry and does not consider the soft tissue envelope. Intra-operative joint tensioning devices allow surgeons to characterize the joint balance before femoral resections, allowing the patient specific soft tissue envelope to be a factor in selecting final component placement. It is unclear however, how pre-operative joint balance is impacted by CPAK classification. Our objective was to characterize the joint imbalance and identify differences between CPAK categories.

Methods

A retrospective review of TKA’s performed using the OMNIBotics platform and BalanceBot (Corin USA, MA) for measuring joint balance using a tibia first workflow were included. Lateral distal femoral angle (LDFA) and medial proximal tibial angle (MPTA) were defined by landmarking after exposure. Joint gaps were measured under a load of 70–90N after the tibial resection throughout flexion. Resection thicknesses were validated to recreate the pre-tibial resection joint balance. Balance was defined as the medial gap minus the lateral gap. Landmarks were corrected for cartilage and bone wear using a similar method to Jenny et al. [3] in which varus outliers were corrected for medial wear (2 mm medial femur and tibial) and valgus outliers for lateral wear (2 mm lateral femur and 3 mm lateral tibia). No posterior femoral wear was corrected.
Knees were subdivided into 9 categories based on the arithmetic hip-knee-ankle angle (aHKA)(MPTA–LDFA) and the joint line orientation angle (JLO)(MPTA+LDFA). aHKA thresholds were defined as >2° Varus, ±2° Neutral and >2° Valgus. JLO thesholds were defined as >3° with a medial distal slope termed ‘Distal Apex’, ±3° from Neutal and >3° lateral distal slope termed ‘Proximal Apex’.
Differences in balance at 10°, 40° and 90° were determined using a one-way 2-tailed ANOVA test with a critical p-value of 0.05.

Results

1124 knees satisfied inclusion criteria. The highest proportion of knees (43.0%) are CPAK I with a varus aHKA and Distal Apex JLO, 80.8% report a Distal Apex JLO and 49.2% report a varus aHKA. Greater medial gaps are observed in varus knees (I, IV, VII) compared to neutral (II, V, VIII) and valgus knees (III, VI, IX) (p<0.05 in all cases) as well as in the Distal Apex groups (I, II, III) compared to the Neutral groups (IV, V, VI) (p < 0.05 in all cases). Reliable comparisons could not be made with the Proximal Apex groups due to low frequency (=1.5%).

Conclusions

Significant differences in joint balance were observed between CPAK groups. A component alignment target based solely on pre-operative boney anatomy may not be sufficient to balance the joint.